Affected versions
Symfony versions >=6.1, <6.4.41, >=7, <7.4.13, >=8, <8.0.13 of the Symfony HTML Sanitizer component are affected by this security issue.
The issue has been fixed in Symfony 6.4.41, 7.4.13, 8.0.13.
Description
Symfony
enumerates the attribute names whose values are scrubbed through
UrlSanitizer::sanitize() (scheme and host allow-lists, javascript:
rejection, BiDi check, etc.). The list is
['src', 'href', 'lowsrc', 'background', 'ping', 'action', 'formaction', 'poster', 'cite'].
Other URL-bearing attributes are absent: <object data=...>,
<applet codebase=...>, <applet archive=...> and
<object archive=...>, <iframe longdesc=...> and
<img longdesc=...>. When an integrator opts these elements/attributes
in via allowElement('object', ['data']),
allowElement('applet', ['codebase']), etc., or via
allowAttribute(), no URL sanitization runs:
data="javascript:alert(1)" and similar payloads ship through unchanged
into the output, enabling stored XSS.
<meta http-equiv="refresh" content="0; url=..."> is the same class of
bug routed differently: the URL is embedded inside a multi-field
content attribute that the per-attribute sanitizer cannot detect from
the attribute name alone. Integrators who enable <meta> with the
content attribute (e.g. via allowStaticElements()) see
content="0; url=javascript:alert(1)" pass through, producing a
refresh-driven navigation to a javascript: URL.
Default configurations are not affected: <object>, <applet> and
<iframe> are not in W3CReference::BODY_ELEMENTS and <meta>
requires an explicit opt-in to <head> context. The vulnerability
surface is integrators who explicitly allow any of those elements together
with the listed URL-bearing attributes.
Resolution
UrlAttributeSanitizer now also routes data, codebase,
archive and longdesc through UrlSanitizer::sanitize(). A new
MetaRefreshAttributeSanitizer registered as a default attribute
sanitizer detects the <delay>; url=<url> syntax inside
<meta content>, sanitizes the embedded URL, and drops the attribute
if the URL is rejected; non-refresh meta content values are passed
through unchanged.
The patches for this issue are available here for branch 6.4 (and forward-ported to 7.4, 8.0 and 8.1).
Credits
We would like to thank Scott Arciszewski for discovering the issue and Nicolas Grekas for providing the fix.